make definitions dir optional for validation

Author: rjernst

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/GenerateTransportVersionManifestTask.java

@@ -34,6 +34,10 @@ public void generateTransportVersionManifest() throws IOException {
         try (var writer = Files.newBufferedWriter(manifestFile)) {
             try (var stream = Files.list(constantsDir)) {
                 for (String filename : stream.map(p -> p.getFileName().toString()).toList()) {
+                    if (filename.equals(manifestFile.getFileName().toString())) {
+                        // don't list self
+                        continue;
+                    }
                     writer.write(filename + "\n");
                 }
             }

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/ValidateTransportVersionReferencesTask.java

@@ -15,6 +15,7 @@
 import org.gradle.api.tasks.CacheableTask;
 import org.gradle.api.tasks.InputDirectory;
 import org.gradle.api.tasks.InputFile;
+import org.gradle.api.tasks.Optional;
 import org.gradle.api.tasks.PathSensitive;
 import org.gradle.api.tasks.PathSensitivity;
 import org.gradle.api.tasks.TaskAction;
@@ -30,6 +31,7 @@
 public abstract class ValidateTransportVersionReferencesTask extends DefaultTask {
 
     @InputDirectory
+    @Optional
     @PathSensitive(PathSensitivity.RELATIVE)
     public abstract DirectoryProperty getDefinitionsDirectory();
 
@@ -39,11 +41,11 @@ public abstract class ValidateTransportVersionReferencesTask extends DefaultTask
 
     @TaskAction
     public void validateTransportVersions() throws IOException {
-        Path constantsDir = getDefinitionsDirectory().getAsFile().get().toPath();
+        Path definitionsDir = getDefinitionsDirectory().getAsFile().get().toPath();
         Path namesFile = getReferencesFile().get().getAsFile().toPath();
 
         for (var tvReference : TransportVersionUtils.readReferencesFile(namesFile)) {
-            Path constantFile = constantsDir.resolve(tvReference.name() + ".csv");
+            Path constantFile = definitionsDir.resolve(tvReference.name() + ".csv");
             if (Files.exists(constantFile) == false) {
                 throw new RuntimeException(
                     "TransportVersion.fromName(\""